The embodiments described herein relate generally to solar power generation, and more specifically, to methods and systems for reducing harmonic current transmitted from an electrical grid to a power generation system.
Solar energy has increasingly become an attractive source of energy and has been recognized as a clean, renewable alternative form of energy. Solar energy in the form of sunlight may be converted to electrical energy by solar cells. A more general term for devices that convert light to electrical energy is photovoltaic (PV) cells.
In order to obtain a higher current and voltage, PV cells are electrically connected to form a PV module. In addition to a plurality of PV cells, the PV module may also include sensors, for example, an irradiance sensor, a temperature sensor, a voltage meter, a current meter, and/or a power meter. PV modules may also be connected to form a module string. Typically, the DC voltages output by the module strings are provided to a power converter, for example, a DC to AC voltage inverter. The DC to AC voltage inverter converts the DC voltage to three-phase alternating current (AC) voltage or current. The three-phase AC output from the DC to AC inverter is provided to a power transformer, which steps up the voltage to produce a three-phase high-voltage AC that is applied to an electrical grid.
Ideally, an AC output voltage of the DC to AC inverter will match the AC voltage on the electrical grid. For example, the AC output voltage of the DC to AC inverter varies sinusoidally at a predefined frequency. When a linear load is coupled to the DC to AC inverter, it draws a sinusoidal current at the same frequency as the voltage. However, when a non-linear load is connected to the DC to AC inverter, it may draw a current that is not sinusoidal. The difference between the voltage and current waveforms may create a harmonic current that is transmitted from the electrical grid to the output of the DC to AC inverter.
Power systems and electrical grids typically include high power electronic loads that may create these harmonic currents. Harmonic currents increase heat in motors, circuit breakers, and transformers, and may cause unwanted equipment trips, circuit breaker trips, and a reduced power factor. As a result, harmonic currents may cause financial losses from added maintenance costs and production down-time. Solutions that passively reduce harmonic currents, for example, a large filter capacitor bank, may also cause undesirable grid resonance. To dampen the resonance created by adding filter capacitors, resistors may be added in series with the filter capacitors. However, an unwanted result of adding resistors is power dissipation by the resistors.